Conventionally, there has been known an optical disc having pre-wobbled pits, as an information recording medium to or from which information is recorded or reproduced, with use of a diffraction limited light spot formed by collecting beams of light on a lens (see e.g. patent literature 1). FIG. 40 is a diagram showing an example of pre-wobbled pits in a conventional information recoding medium disclosed in patent literature 1.
Referring to FIG. 40, the following method is known. Pits 901 are wobbled and formed in grooves 902, and in gap portions between the grooves 902 adjacent to each other. A synchronization signal is generated by the pits 901 in an area 903. The synchronization signal is compared with a reproduction signal to be output at a timing corresponding to the pre-wobbled pits 901 within an area 904 for checking the center of the track. Then, for instance, an offset of a tracking signal to be obtained from the grooves 902 is corrected. Information is recorded in an area 905 and thereafter. Further, a plurality of the areas 904 each having the pre-wobbled pits 901 are formed in the periphery of the optical disc. Information in the areas 904 is sampled and held, and used as a tracking signal.
The above conventional technology employs a phenomenon that the reflection efficiency varies depending on a positional relationship between a light spot and a pit, when the light spot passes pits each having a predetermined depth, or pits having depths different from each other. A change in the light amount of light that returns as reflected light is read as a reproduction signal. A light spot is formed by collecting light emitted from a laser light source on an objective lens up to the diffraction limit. The size of the light spot is determined based on the wavelength λ of light and the numerical aperture NA, and is about λ/(2 NA). For instance, in the case where λ=410 nm and NA=0.85, the size of the light spot is about 0.24 μm. In an information recording medium using such a light spot, the track pitch Tp is set to about 0.32 μm, the size (diameter) of a wobble pit is set to about 0.24 μm, which is substantially equal to the size of a light spot, and the wobbling amount is set to about Tp/4 (0.08 μm), for instance.
However, in a conventional configuration, the size of a light spot is restricted by the diffraction limit that is determined based on the wavelength of light and the numerical aperture. In order to reduce the light spot for the purpose of enhancing the recording density of information, it is necessary to shorten the wavelength of light or to increase the numerical aperture. Light of an extremely short wavelength is ultraviolet light. Therefore, it is difficult to implement a light source, the material of optical components is limited, and it is impossible to use the optical components with ease. It is true that the numerical aperture larger than one can be implemented, with use of SIL (Solid Immersion Lens) or a like member. However, the numerical aperture is multiplied only by a multiple substantially as large as about a refractive index. Therefore, there is a limit on the size of the numerical aperture.
As described above, it has been difficult to reduce the size of wobble pits, and it has been impossible to utilize the wobble pits, as means for detecting a tracking signal in a higher-density information recording medium.